Detecting Evolutionary Changes to Stressors

Anthropogenic disturbances such as pollution, habitat modification and destruction, climate change, and species exploitation are impacting on the distribution and abundance of many sensitive species. Monitoring and predicting the ability of populations to respond to different environmental stressors is vital to understand and minimize their impact on species (see Ecotoxicology: The Focal Topics; Exploitation; Fire; and Climate Change 3: History and Current State). Individuals respond to stressors in various ways, depending on intensity and duration. Below we discuss how stress responses can be detected and provide examples of how stressors are already influencing natural populations.

Evolutionary Responses Detected through Traits

Populations have the potential to evolve and thereby shift phenotypes over short periods of time. Impacts of stressful environmental conditions on populations can be detected through phenotypic shifts, such as increases in tolerance levels, changes in the timing of life history traits including flowering and reproduction, or changes in morphology. These phenotypic shifts can provide direct evidence of rapid responses to stressful environments and there are many examples in the literature. These include the many cases of rapid evolution of pesticide resistance in a variety of insects and mites to a range of chemical classes such as cyclodienes, carbamates, formamidines, organopho-sphates, and pyrethroids. Resistance is often first detected from the failure of a pesticide to control a pest species and this is usually validated by experimentally comparing tolerance levels of putative-resistant and sensitive populations. Resistance has also evolved in organisms not targeted by pesticides; for example, natural populations of the vinegar fly D. melanogaster are resistant to insecticides such as dichloro diphenyl trichloroethane (DDT) even though they have never specifically been targeted for control by these pesticides.

Evolutionary responses to pollutants have been less widely documented than responses to pesticides. Nevertheless, there are now well-studied cases of plant species evolving in response to waste from mining operations. Plant populations growing close to smelters commonly show a much higher level of resistance to heavy metals than other populations. The evolution of pollution tolerance has also been demonstrated in aquatic fauna, including crayfish, worms, and midges. However not all plants or animals successfully evolve in response to pollutants: levels of biodiversity are often much lower in polluted compared to unpolluted areas, suggesting that many species are unable to adapt.

Pollution stressors can indirectly influence the evolution of traits. There is the classic case of melanism in the peppered moth evolving due to increased pollution levels in England in the 1800s. Although this species of moth is usually a light color, a darker melanic form increased in frequency in some polluted areas during this time. The increase in this form was not a direct consequence of pollution but a response to predation by birds; pollution caused the trees which these moths reside on to darken and the melanic form was less likely to be predated upon than the previously camouflaged lighter form. This example highlights that evolutionary responses can reflect changes in biotic interactions arising as a consequence of an environmental stress as well as its more direct effects.

Phenotypic evidence is accumulating on organism responses to recent climate change. Average global surface temperatures have risen 0.8 °C in the past century and a rapid 0.2 °C per decade over the past 30 years, and several studies that span decades are linking these changes to shifts in the timing of life history traits and geographical shifts in species ranges. A variety of birds, butterflies, and alpine herbs in the Northern Hemisphere have altered their geographic boundaries; a meta-analysis on 99 different species found significant range shifts, averaging 6.1 km per decade away from the equator. Additionally, a 25 year study of 65 bird species in the United Kingdom found that 20 species are now first laying eggs an average of 8.8 days earlier in 1995 than in 1971.

To test whether these patterns reflect evolutionary shifts rather than just phenotypic variants of the same genotype (phenotypic plasticity; see Phenotypic Plasticity), a genetic basis needs to be established through controlled breeding experiments or family studies. Changes in the timing of diapause induction in the pitcher plant mosquito is an example of adaptive evolution in response to longer growing seasons induced by recent climate change. Parent-offspring comparisons have shown the timing of diapause induction in the pitcher plant mosquito has a heritable genetic basis and this trait has also been shown to cline with latitude along eastern North America. When southern and northern populations are exposed to mid-latitude lengths, they enter diapause either too late (southern) or too early (northern) and experience between a 74% and 88% decline in fitness, indicating that diapause timing is an adapted phenotypic response. Over the last 30 years, this latitudinal cline has shifted, with populations from the north shifting toward a more southern shorter day length form as growing seasons have become longer due to warmer temperatures. Shifts in size and movement patterns in small mammals and some bird species have also been linked to evolutionary shifts in response to climate change.

However, the rigorous experimental approach needed to demonstrate a genetic basis for a phenotypic shift cannot be readily carried out for many organisms, particularly when they have long generation times. For this reason much of the recent attention on detecting evolutionary changes under stress has moved to the genetic level.

Evolutionary Responses Detected through Allele Changes

Underlying evolutionary changes are allele frequency changes - alleles that are at a selective advantage in particular environments are favored and increase in frequency. As evolutionary responses to stress directly involve changes in allele frequencies, detecting changes at this level is potentially the most direct way to measure and predict the biological consequences of stressors in populations. Initial studies used neutral markers to compare levels of genetic diversity between polluted and nonpolluted populations was one of the first ways DNA markers were used to assess the genetic impacts of stress. However, these markers do not reflect adaptive changes unless they are closely linked to a gene under selection. Genetic markers based on loci coding for proteins (allozymes markers) have been used in some studies, but these can only be applied to well-preserved specimens, which limits their application in longitudinal comparisons of specimens that are often poorly preserved.

One example where changes in allozyme markers were successfully linked to climate change is the alcohol dehy-drogenase enzyme (Adh) in D. melanogaster. In the 1980s, the allele frequencies of the Adh gene (AdhF and AdhS) were found to cline with latitude across the continents of Asia, Australasia, and North America. This association with latitude has been implicated with thermal tolerance where flies with the AdhS allele are more likely to survive heat shock than flies with the AdhF allele. The rapid formation of these clinal patterns across continents highlights strong climatic selection on these genes. To determine whether recent changes in climate have influenced the clinal patterns in this marker since the 1980s, the Australian east coast cline was reinvestigated in 2000 and 2002. The slope of the latitudinal association had not changed over the 20 year study period; however, a shift equivalent to 4° in latitude was detected in the height of the graph, with the southern high latitude populations now having the genetic constitution of the more northerly populations. Frequencies of the chromosome inversion polymorphism (In(3R)P) in D. melanogaster have also changed drastically in the last 20 years (Figure 2). Furthermore, changes in chromosome arrangement frequencies of D. robusta from North America have been linked to shifts in minimum temperature since the 1970s. Similar parallel shifts in latitude association in numerous inversion polymorphism analyzed simultaneously have also been detected in

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Figure 2 Change in latitudinal patterns, between 1979 and 1982 and between 2002 and 2004, of the inversions In(3R)Payne Open symbols and dashed lines indicate 1979-82 pooled data solid symbols and solid lines indicate 2002-04 pooled data. From Umina PA, Weeks AR, Kearney MR, McKechnie SW, and Hoffmann AA (2005) A rapid shift in a classic clinal pattern in Drosophila reflecting climate change. Science 308: 691-693. Reprinted with permission from AAAS.

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Figure 2 Change in latitudinal patterns, between 1979 and 1982 and between 2002 and 2004, of the inversions In(3R)Payne Open symbols and dashed lines indicate 1979-82 pooled data solid symbols and solid lines indicate 2002-04 pooled data. From Umina PA, Weeks AR, Kearney MR, McKechnie SW, and Hoffmann AA (2005) A rapid shift in a classic clinal pattern in Drosophila reflecting climate change. Science 308: 691-693. Reprinted with permission from AAAS.

populations of D. subobscura across three separate continents. These studies reflect the power of latitudinal markers to act as sensitive indicators of climatic change.

Ideally, shifts in allele frequencies in genes specific to particular environmental stressors could be used in the future to track genetic responses to stress. Some progress has been made in the area of pesticide resistance. Microarray technology has led to the discovery that a single chromosome P450 gene, Cyp6g1, confers resistance to DDT in D. melanogaster. Cyp6g1 was found to be over-expressed in D. melanogaster lines from natural populations that exhibit resistance due to the insertion of a transposable element in the promoter region that regulates expression of the gene. Examination of the patterns of molecular variation around this gene showed a sharp reduction in the level of molecular variation in this area of the genome indicative that it had undergone recent selection (selective sweep). Under a selective sweep, variation decreases in the genomic region around the candidate gene, as alleles at adjacent loci spread with the new allele favored by selection. Associations between insecticide resistance and specific gene changes have been identified for other pesticides in D. melanogaster and in the sheep blowfly, Lucilia cuprina, as well as for heavy metal tolerance in the mosquitofish.

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